Machining CR39...

[Dontech]

Good morning, all.

Anyone have any experience machining CR39 (allyl carbonate) plastic? We are looking at a potential new job using this material and I need to put a game plan together. We last ran this material @ 20 years ago. Our bread and butter tools back then were HSS and cobalt, but they just didn't hold up very long. Tried both coated and uncoated carbide and tool life went up but finish quality was kinda lackluster, to be kind. We eventually tried some PCD endmills and tool life went up dramatically, but our finish quality still disappointed. Our best machine at the time maxed-out at 6,000 rpm so not only were we RPM-limited, we also have to run everything we machine dry...meaning we didn't get the potential advantage of lubricating the tool or the cut. Within those parameters we just never found our sweet-spot between tooling, RPM, and feed-rate; the only saving grace being the job was just an OD cut, meaning the less-than-ideal finish wasn't visible after assembly. With this new job, though, we have the added complication of a step-cut so now finish quality becomes much more important. 

There isn't any actual surface-finish requirement on the job but even 20 yrs later the unimpressive results using PCD tools still chaps my hide so I'd like to do much better this time around.  We've got a max of 15k RPM and a whole slew of more modern tool materials and geometries available now so I'm hoping that one or more of you might have a tip or two for me, re: feeds, speeds, and tool suggestions.

[AHarrison1]

I generally have good results using carbide 3 flutes and avoid the temptation to run as fast as possible. i try to stick to about 700 SFM,

faster than that could 'burn' your material

Chipload will depend on the size of the cutter but I would go about .0005" to .001" less per tooth than ALU.

[Dontech]

Thank you for the feedback.  I will give those suggestions a try.